Method of and apparatus for heattreating



J. w. HARscH 1,949,716

Fueauarch 1o, 1932 s sheets-sheet 1 METHOD OF AND APPARATUS .FOR HEAT TREATING March s; 1934.

INVENTOR lf- M 1 ATTORNEY March 6, 1934. J. w. HARscH METHOD OF AND APPARATUS FOR HEAT TREATING Filed March l0, 1932 3 Sheets-Sheet 2 Illl ATTORNEY E INVENTOR L im 1 z z 2 2 Marchl, 1934. ,1 w HARSCH 1,949,716

METHOD OF AND APPARATUS FOR HEAT TREATING Fild March l0, 19252 5 Sheets-Sheet 3 ///l 59@ Q4@ Su@ f/ )j *25 52 -25 32. 25a 52 25152 ifi Q f l@ H f@ Q //////////////////////////;/j /I 7 Ffa-9 )4 ATTORNEY Patented Mar. 6, 1934 UNITED STATES PATENT OFFICE METHOD F AND APPARATUS Fon HEAT- 'rnEA'rING 21 Claims. (01.266-5) My invention relates to method of and appara.- tus for heat-treatment of metal objects, as drawing, hardening, tempering, or the like.

In accordance with my'invention, the atmosphere in a furnace is forcibly circulated in a path intercepting the path of movement of the load through the furnace to obtain uniformity of temperature for any given section across the load. and more particularly, also to transfer heat by l0 A convection to the load.

Preferably, a series of fans are disposed along the path of movement of the load, to ensure forcible stirring' of all air contacting with the load.

More particularly, the work heating chamber may be divided, as by suitable bailles, into zones of definitely different temperature, with at least one fan in each zone, or the baffles may be omitted so that the zones merge. the temperature progressively increasing toward the furnace outlet. further in accordance with my invention, the ratio of heat transferred by convection to heat transferred by radiation is made as high as practically possible, as by interposing shields between the work and the source of heat, and by increasing thearea of the surface washed by the circulating gas for absorption of heat.

Also in accordance with my invention, to compensate for decreased efficiency of transfer by convection at the higher temperatures, for example, at zones more adjacent the furnace outlet, the speed of the fans may be there'increased to pass a greater weight of air per unit of time into heat transfer relation with the work and the source of heat; the air may be confined to preclude decreased density at the higher temperature; and/ or the area of the heating surfaces for thel circulating gas may be increased.

My invention further resides in the methods and systems hereinafter described and claimed.

For an understanding of my invention and for some of the various forms it may take, reference is to be had to the accompanying drawings in which:

Fig. 1 is a sideelevational view, in section, of a continuous furnace utilizing the invention.

' Fig. 2 is a front elevation, in section, of the furnace shown in Fig. l.

x Fig. 3 is a top plan view. with parts removed. 'and parts in section, of the furnace shown in Fig. 4 is a side elevational view of another type of continuous furnace constructed in accordance with the invention. 1 f

Fig. 5 is a front elevational view, in section, of the furnace shown in Flix. 4.

Fig. 6 is a plan view, partly in section, of the furnace shown in Fig. 4.

Fig. 7 is a detail'view, on enlarged scale, of the arrangement of heater elements.

Fig. 8 is a side elevational view, in section, of 60 a continuous furnace having definite temperature zones.

Fig. 9 is an end elevational view, in section.- of a fixed furnace embodying my invention.

Referring to Figs. 1, 2 and 3, metal objects. such 65 as gears, shafts, valves andthe like to be heated, are permitted to slide down the chute 1 at the inlet end of the furnace 2 which discharges them on the perforated conveyor chain or belt 3 driven by motor 4 through a suitable speed-reducing 70 means 5. The door D, or equivalent, permits enftry of the articles into the furnace while normally substantially isolating the furnace chamber from the outer atmosphere. At the outlet end of the furnace, the objects which have been heated to 'f5 the desired temperature by their passage through the chamber 6, are discharged into the quenching bath 7.

The plate or trough 8 which supports the upper lift of the conveyor chain, and the plate 9 which so supports the lower lift thereof, are perforated for reasons hereinafter explained.

The heating elements 10, specifically electric resistor elements, are disposed along the side walls of chamber 6 for transfer of heat to the g5 objects or material being transported by the conveyor belt or chain. The heat may of coursebe obtained from other heating sources, as gas burners, oil burners, etc.

In the system thus far described, the objects at the edges of the conveyor belt are heated by radiation from the resistors, and the other objects of the load are heated by conduction from them.

, With that arrangement, it is practically impossible to obtain uniformity of temperature unless gg a. very long soaking period is allowed.

In accordance with the present invention, the load is raised to the desired temperature in shorter time with uniformity of temperature over each and every cross-section ofthe load by disposing a plurality.y of fans 11, 11a above and/or below the material being conveyed. The fans, as shown, .are preferably individually driven, as by electric motors M.

During passage of the work through the cham- 1,05V ber, preferably all of these fans are running to effect transfer of heat by forcible circulation of air, or the like, between the source of heat and the objects under treatment. To increase the ratio of heat transferred by convection to the etc.

heat transferred by radiationythe shields 12 of suitable insulating material, as insulating firebrick, are interposed between the heater elements and the furnace load. 'Ihe shields may also be of refractory material, or spaced walls of metal, 'I'he shield 12 may be spaced so that the air is free to circulate on both sides thereof for transfer of heat thereto. So disposed and constructed, the shield 12 has a two-fold benecial effect upon the ratio of convection heat to radiant heat; it not only intercepts some of the radiant heat that would otherwise be received by the work but it transfers this heat to the circulating heat vehicle so that more heat is transferred by convection.

With the heaters controlled so that all are of the same temperature, and with the fans at the outlet end running at higher speed, the temperature difference between the heaters and the work is greatest at the inlet end of the furnace, and the rate of transfer of heat by radiation would ordinarily be much greater than at the outlet end of the furnace, because the rate of heat transferred by radiation varies as the difference of the fourth power of the absolute temperatures of the radiating and absorbing bodies. Y

To compensate for the materially greater temperature difference at the inlet end of the furnace, the shield l2, by construction, or selection of material is made to be of greater effectiveness at that end of the furnace, for example, itmay be of greater thickness.

Further, to improve the ratio of convected heat -to radiant heat, the heater elements `along each of the side walls, may be disposed in a metal container or cylindei` 13 provided with fins 14 or the like, for increasing the area of the heated surface washed by the forcibly circulated air.

If on the otherv hand, all of the fans are run at the same speed and the heater sections are .not controlled to run at substantially the same temperature, the heaters at the cold end of the furnace run colder because more heat is removed from them and transferred to the work by the circulating air than is removed from the heaters, or equivalent, at lthe hot end of the furnace. Under these circumstances, it is desirable to have the shield of greater thickness, or greater effectiveness at the hot end of the furance. l

Referring to Fig. 2, assuming that the upper fan 11 is running as a suction fan and the lower fan 11a is running as a'blower with respect to the central or work chamber 15, air is drawn through the air heating chambers 6a, 6b defined by the partitions or walls 16, of metal for example, extending lengthwise of the furnace and bracedor supported by members S, into heat transfer relation with the heated structure 13, fins 14, shield 12, etc., all of which are effective in transfer of heat to the air. 'I'he heated air is then blown by the lower fan 11a through the perforations of plates 8 and 9, through the conveyor chain 3 into and out of contact with the objects carried by the chain. Therefore, throughout the length of the work chamber, the material is heated not only by radiation but very materially by convection. The load upon the conveyor is relatively shallow so that the transfer by radiation is minimized, and since the load is shallow and of relatively great cross-section, insubstantial resistance is offered to the circulation of the convection air.

Preferably, the fans are operated so that their air streams form more or less clearly defined heat transfer zones; for example, all of the fans may be running in such direction that the all l'cnlated by th individual pairs of fans flows in the manner indicated in Fig. 2. If adjacent fans on.

. until equilibrium is reached.

At the higher temperatures, the transfer of heat by convection decreases in efficiency because of the decrease in density of the convected gas. This can be offset by increasing the fan speed, but in practice this is not very feasible for the operating temperature of the furnace may be so high that the rotating parts have not sufficient strength at the elevated temperature to withstand the stresses imposed by high speed; further, the fan eiiciency falls olf because the slip rapidly increases with increased speed. In furnaces generally of the type disclosed in my prior Patent 1,578,027, the efficiency of transfer by convection may be improved at the higher temperatures by preventing escape of air, so that the weight of air circulated per unit of time is substantially always the same. 'Gases or vapors of high specific heat, as mercury, may be used in high temperature operations for improved convection transfer.

In a furnace of the type herein shown, the decreased density of 'the gas at the higher temperatures is most readily compensated by increase of the temperature or area of the surfacev or surfaces from which the gas receives heat; for example, the fins 14 may be increased in number or area to afford transfer of more heat by convection to the circulating gas at the outlet end of the furnace.

The furnace shown is not only suitable for simple heat treating but also for processes which involve a chemical change, for example, nitriding, carburizing, bright annealing, etc.

In general, the furnace construction shown in Figs. 4 to 6 is similar to that described, and the same reference characters are applied to the corresponding elements. v In this type furnace the Work or load is placed in relatively shallow trays or baskets 17. Usually, these trays extend the length of the heating chamber within the furnace, 'and when an additional tray is moved into the furnace from the loading platform L, as by the motor-driven pushing mechanism 18, the tray at the other end of the furnace is pushed out onto the unloading platform U. During this operation the doors 19 and 20 at the inlet and outlet ends of the treating chamber are, of course, open. As a new tray is inserted, each of the baskets or trays within the furnace is advanced one position nearer the outlet.

At desired intervals, the motors 21 are energized to open the doors of the furnace, the push motor 18 is energized to force in a new basket of work, and to force out a basket which has attained the desired nal temperature. The heat treatment therefore consists of successive stages, the work advancing step by step through the treating chamber instead of continuously as in Fig. 1, though obviously in the former modification, the motor 4 may be intermittently energized at suitable intervals to effect step by step advance of the load.

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`fans, and terminate just above the load.

furnace of the tray type vshown in Fig. 4 is c m- The bottoms of the trays 17 are perforated, as shown, so that the air circulated by fans 11 and/or 11a passesthrough them, into and out of contact with the work carried thereby.

The furnace lmay be operated as a batch furnace, that is, the trays are introduced at the same time and at the expiration of a predetermined time, or upon attainment of a given temperature all are removed from the furnace. The surface of the load presented to receive heat by radiation is small while the surface presented to the circulating gas is large, hence, the ratio -of heat transferred by convection to heat transferred by radiation is desirably high. As, with batch operation, all parts of the furnace are at the same temperature, there is no need for tapering of the baffles.

Similarly, the furnace of Fig. l may be operated as a batch furnace by using the conveyor only to introduce the load at the beginning of a run and to remove it at the end of the run.

In this modification, the -heater elements 10 formed by electric resistance conductor suspended from the side walls of the furnace are exposed to direct contact with the circulating air. As in the prior modification, the shielding structure 12 may be used to limit the transfer of heat by radiation, and as shown, the shield, eitherby selection of material, or by increased thickness, is more effective at that end of the furnace where the difference in temperature between the heat and the work is greatest.

Alternatively, as shown in Fig. '1, or in addition, the turns of resistor 10 at the inlet end of the furnace may be more widely spaced than at the outlet end to reduce transfer by radiation. The same purpose may be accomplished by dividing the resistor v10 into sections, and operating them at different temperatures. For example, the sections morev adjacent the inlet end of the furnace may be operated at temperatures lower than the section or sections at the outlet end of the furnace. In a gas fired furnace, the same general result may be obtained by controlling the individual burners.

For increasing the transfer by convection, the corrugated sheet metal pieces 22 may b'e disposed in the air heating chamber, for example, in contact with the shielding structure 12. As shown, the corrugations run parallel to the direction of air flow, so that in. effect there are formed a multiplicity of ducts whose walls are effective to transfer heat to the gas, these walls receiving heat by conduction from the shield, and by radiation from the source of heat, and also in some cases from the shield itself, when the latter is of such material that under the conditionsof operation of the furnace it becomes a secondary source of radiant heat.

As in the arrangement of Fig. 1, the load'mass, particularly as a whole, is shallow to present only a relatively small area in position to be viewed by the radiant heat source, and to afford small resistance to the circulation of air. This conditionis favorable to attainment of a high ratio of `heat transferred by convection to heat transferred by radiation.

In the modification shown in Fig. 8, which is generally similar to Fig. 1, the heatingchamber is divided into denite zones of different temperature by the partitions or walls 25, 25a which depend from the roof of the chamber between the If a ployed, the lower edges of these partitions should just clear the tops of the trays.

As the load passes each of these walls, it may move into a zone of definitely higher temperature, and all parts of the portion of the load in each zone are uniformly heated to the same temperature by the air forcibly circulated in the zone. The ratio of heat transferred by radiation to the heat transferred by convection can be controlled for each zone by selection of fan speed, radiant heat baffles, area of heat transfer surfaces washed by the circulated gas, etc., as previously explained.

If it is desiredto hold the work at a certain temperature for a longer time than afforded by one zone, the next one or more zones may be held at the same temperature.

The heat energy of the different zones may be individually controlled. For electric heating, to which the invention is not limited, the heater sections for the'several zones may be individ-4 ually supplied by the pairs of feeders 26 to 30, etc., and the supply for each section is regulated by a controller 3l actuated by a temperature-responsive device 32, as a thermocouple. Each controller is set to maintain the temperature of the associated zone at a desired value. As the temperature rises above that value, the heat supply is reduced or interrupted and vice versa. For gas fired furnaces, the controller 31 would regulate Ll the fuel valve, or otherwise modify the combustion rate.

For some processes, for example nitriding, it is desirable, as described and claimed in co-pencling application Serial No. 599,052, filed March 113 15, 1932, to hold the load temperature at one or more low values for a predetermined time, and thereafter progressively to increase the temperature. This can be accomplished by providing one or more 10W temperature zones, as by partitions 25 of Fig. 8, with omission of partitions 25a. The heaters for the last zone, of progressive temperature rise, may be controlled as a unit from a single thermocouple, preferably the one nearest the furnace outlet. lzt

The fans ensure uniformity of temperature throughout the work in the low temperature zones, uniform rise as the work approaches the furnace outlet, and uniformity of final temperature throughout the Work leaving the heating 11:5 chamber.

The furnace shown in Fig. 9 is generally similar to those previously described except that it is fired by gas, vapor, or liquid. The nozzles 33 supply fluid fuel, or a mixture of fuel and air, 130 to the combustion chambers 34 whose walls 35 transfer heat to the Work by radiation and which are washed by the circulating air for transfer of heat by convection. The combustion gases are removed by the iiues 36. In some treatments, as carburizing, at least some of the gases may be introduced intol the heating chamber by opening valve 37 to suitable extent. Valve 38 may be opened to permit escape through duct 39 of gas 1 displaced from the heating chamber by the incoming' fresh gas. The temperature may be controlled as previously described to obtain different values for different zones, and valves 37, 38 per. mit different gas concentrations in th different 4.. zones for the different temperatures. o

For brevity in the appended claims, the term air is used in a generic sense to comprehend the furnace atmosphere whether it be a gas, or vapor, or any mixture thereof and whether chemi- 150 cally active or neutral'.

What I claim is:

1. In the art of heat treatment, the method which comprises increasing` the temperature o1' material being treated by passing it through a series of zones containing a radiant heat source, in each of said zones, forcibly circulating a gas to transfer heat to said material by convection,

and limiting the transfer by radiation in said zones to maintain substantially high the ratio of heat transferred to said material by convection to the heat transferred to said material by radiation.

2. In the art of heat treatment, the method which comprises passing-the material to be treated through a series of zones, in each of said zones transferring heat to saidtmaterial by radiation, and in each of said zones and substantially independent of the other zones, forcibly circulating a gas past the source of radiant heat and then past said material to transfer heat to said material by convection.

3. In the art of heat treatment, the method which comprises passing the material to be treated through a series of zones, in each of said zones heating the material by indirect radiation from a source of heat, and by forcible circulation of a gas; and, in at least the higher temperature zones, increasing the convection transfer, by heating from said source, surfaces washed by the circulating gas.

4. In the art of heat treatment, the method which comprises passing the material to be treated through a series of zones, in each of said zones heating the material by indirect radiation from a source of heat and yby forcible convection of a gas; in at least the first one or more of said zones, intercepting a substantial portion of the radiant heat to reduce the transfer of heat to said material by radiation, and utilizing the intercepted heat energy for heating the convected gas 'to increase the transfer of heat to the material by convection.

5. A heat-treating system comprising a workheating chamber, structure for conveying material to be treated through said` chamber, a source of heat, and a plurality of fans spaced in the direction of movement of said conveying structure for circulating air heated from said source into and out of contact with the material.

6. A heat-treating system comprising a Workheating chamber, air-heating chambers, said work-heating chamber and containing a source of heat, structure for conveying material to be treated through said first chamber, and a plurality of fans, each effecting circulation of air between air-heating and work-heating chambers and forming different zones of heat transfer to material being conveyed by said structure.

'7. A heat-treating system comprising a workheating chamber, air-heating chambers lateral to said work-heating chamber and containing a source of heat, structure for conveying material to be treated through said first chamber, a plurality of fans each effecting circulation of air between air-heating and work-heating chambers and forming different zones of heat transfer to material being conveyed by said structure, and shielding structure disposed between said source of heat and the material in at least one of said zones to reduce the transfer of heat by radiation.

8. A heat-treating system comprising a workheating chamber, air-heating and Work-heating said work-heating chamber and containing a source of heat, structure for conveying material to be treated through said first chamber, a pluldisposed in at least veying material to be treated along said chamvvof said chamber,

tending along said lforated bottom, and a plurality of fans spaced. l5@

rality of fans each effecting circulation of air between said chambers and forming different zones of heat transfer to material being conveyed by said structure, and shielding structure disposed between said source of heat and the material to reduce the transfer of heat by radiation and havmg greater effectiveness. at different regions to compensate for the greater difference in temperature between the material and the source of' heat.

9. A heat-treating system comprising a workheating chamber, foraminated structure for conveying material to be heated through said chamber, a source of heat, and fans disposed along the path of movement of said structure for forcibly circulating alr heated from said source through said structure and into and out of contact with the material being conveyed thereby.

10. A heat-treating system comprising a workheating chamber, foraminated structure for conveying material to be heated through said chamber, a source of heat, and a plurality of fans disposed along the path of movement of said structure above and/or below the same for forcibly circulating air from said source of heat through wir said structure and toward and away from the 'material being conveyed thereby.

11. A heat-treating system comprising an elongated work-heating chamber, a chamber substantially co-extensive therewith, a source of heat within said second chamber, passages connecting the upper ends of said chambers substantially throughout their length, passages connecting the lower ends of said chambers substantially throughout their length, foraminated 11m structure for conveying material to be treated along said work chamber intermediate said upper and lower passages; and a plurality of fans one group of said passages for forcibly circulating air in closed paths through 1M; said chambers and passages which intersect the path of movement of said conveyor structure.

12. A heat-treating system comprising a workchamber, a second chamber containing a source of heat, means for forcibly circulating air through lill] said chambers for convective transfer of heat to the work, a shield within said second chamber for intercepting radiant heat from said source, and sheet metal structure disposed within said second chamber to be washed by the air for increased transfer of heat by convection.

13. A heat-treating system comprising a worlrheating chamber, structure for conveying material to be treated along said chamber, and a plurality of fans spaced along said chamber for 13@ forcibly circulating the air in said chamber into and out of contact with said material.

14. A heat-treating system comprising a Workheating chamber, foraminated structure for conber, and a .plurality of fans spaced along said chamber above and below said structure.

15. A heat-treating furnace comprising a chamber, heater elements along the side walls of said chamber, a plurality of shallow trays for l transporting material along. said chamber and having perforated bottoms, and a plurality oi? fans spaced along said chamber for circulating air through said trays and into and out of contact with said material.

16. A heat-treating furnace comprising a chamber, heater elements along the side walls a shallow work trough exchamber and having a peralong said chamber for circulating air through said trough.

17. A heat-treating furnace comprising achamber, heater elements along the side walls of said chamber, a perforated plate extending along said chamber, a work-conveying chain supported by said plate, and a plurality of fans spaced along said chamber for circulating air through said plate and chain.

18. A heat-treating system comprising a workheating chamber, foraminated structure for conveying material to be treated along said .chamber, and a plurality of fans spaced along said chamber above said structure.

19. A heat-treating system comprising a workheating chamber, foraminated structure for conveying material to be treated along said chamber, and a plurality of fans spaced along said chamber below said structure.

20. Al heat-treating system comprising a workheating chamber, structure for dividing said chamber into definite zones, means for conveying material to be treated through said zon in succession, means for supplying heat, and a plurality of fans so disposed that at least one fan is associated with each of said zones for forcibly circulating air from said source through the zone.

2i. A heat-treating system comprising an elongated work-heating chamber, means for transporting work to be treated along said chamber, a source of radiant heat, shield structure interposed between the work and said source of heat and of different eiectiveness for diierent regions lalong said chamber, and a plurality of fans for forcibly circulating air between said chamber and said source of heat in a plurality of paths intersecting the path of movement of the work along said chamber.

, JOHN W. HARSCH.

liso 

